The present invention pertains to compositions for making packaging film, and more particularly to a thin polymeric film suitable for replacing polyvinyl chloride film especially in packaging applications.
Polyvinyl chloride (PVC) has long been used in many applications in the packaging art. One particularly widespread application for PVC is the use of such material as an overwrap material for trayed retail cuts of meat and other food products in a retail environment such as a supermarket.
PVC has several desirable properties for this use. For example, it has excellent burn-through resistance, optics and good elasticity and stretch properties at use temperatures.
Unfortunately, PVC also has several disadvantages, including the production of hydrogen chloride gas during heat sealing and the generally corrosive effects of such gases in the packaging room. Extractables from the PVC into the packaged food product have also become of concern.
It would be of great benefit to the packaging industry, and particular to applications requiring an instore film for overwrapping trayed food products, to provide a film with many of the advantages of PVC but without the disadvantages described above.
In addition to the foregoing, it is desirable to provide a film as aforesaid without the disadvantages of PVC, and to provide such a film which is cross-linked without irradiation. In the applications described above the film material is wrapped around the product and sealed as on a hot plate. Without cross-linking the film might burn through on the hot plate and therefore cross-linking is desirable to provide high heat resistance. Irradiation is customarily used to provide cross-linking, but this procedure is inconvenient, expensive and inefficient and also difficult to perform on very thin films.
It is also desirable to provide such a film which is stretch oriented to provide improved properties. Processes for producing oriented films and oriented films themselves are known in the art.
U.S. Patent No. 3,456,044 (Pahlke) mentions thin films of thicknesses less than 1 mil such as 0.5 mils, and discloses a double bubble method for biaxially orienting thermoplastic films, including the steps of producing a primary tubing which is inflated by introducing air into the interior thereof, and a cooling ring 22, as well as squeeze rolls 34 and 28, with rolls 34 having a greater speed than rolls 28. Between the two pairs of squeeze rolls is a reinflated secondary bubble. If annealing is desired, the tubing can be reinflated to form a bubble 70.
U.S. Pat. No. 3,555,604 (Pahlke) is a patent based on a divisional application which was derived from the same priority application as the '044 patent described above, and discloses the same information described above for the '044 patent.
U.S. Pat. No. 4,258,166 (Canterino et al.) discloses a uniaxially oriented plastic film material with improved strength and clarity in the direction of orientation preferably comprising homopolymers and copolymers of ethylene.
U.S. Pat. No. 4,355,076 (Gash) discloses monoaxially oriented polypropylene film laminated to a monoaxially oriented high density polyethylene film, the films produced by for example tubular blowing.
U.S. Pat. No. 4,440,824 (Bonis) discloses a thermoformable coextruded multilayer structure useful for thermoforming into containers, the structure having polyolefin coextruded with a high impact polystyrene layer. A five layer structure is shown.
U.S. Pat. No. 4,464,439 (Castelein) discloses a coextruded laminate having a sheet of polypropylene and a sheet of a mixture of high impact polystyrene, crystalline polypropylene, and styrene/dienic monomer block copolymer.
U.S. Pat. No. 4,879,177 (Boice) discloses a monoaxially oriented shrink film having a core layer of butadiene styrene copolymer, outer layers of ethylene propylene copolymer, and intermediate bonding layers of ethylene copolymer.
U.S. Pat. No. 5,219,666 (Schirmer et al.) discloses polymeric, oriented films made by the use of a combination of a hot blown process and a blown bubble process.
It is an object of the present invention to provide a cross-linked, stretch oriented film which can be included in a thermoplastic film useful as an overwrap material for trayed retail cuts of meat and other food products in a supermarket or other retail environments.
It is a further object of the present invention to provide a film as aforesaid that can be cross-linked without irradiation.
It is a still further object of the present invention to provide a film as aforesaid which has good burn-through resistance.
It is an additional object of the present invention to provide such a film with other desirable characteristics, such as good optical properties.
Further objects and advantages of the present invention will appear hereinbelow.